The present invention generally relates to superconducting devices and more particularly to a Josephson device that has an overlayer structure.
The Josephson devices are characterized by an extremely high operational speed and low power consumption, and are expected to play a major role in future super-fast computers and other digital devices. Further, the Josephson devices are extremely sensitive to magnetic fields and are used for analog applications, such as magnetic measurements, by constructing the superconducting quantum interference devices (SQUID).
Generally a Josephson junction is required to satisfy the following conditions:
(1) the Josephson junction should have a barrier layer that covers the lower superconducting electrode without forming a pinhole; and PA1 (2) the barrier layer should be an excellent insulator and be stable against various heat treatments applied during the fabrication process of the device. PA1 (1) The aluminum overlayer should cover the underlying niobium superconducting electrode with a thickness less than several nanometers in correspondence to the coherent length in the niobium electrode. PA1 (2) The AlO.sub.x barrier layer formed by the oxidation of the surface of the aluminum overlayer should have an excellent thermal stability. Further, the AlO.sub.x layer should be an excellent insulator. PA1 (3) The boundary between the aluminum overlayer and the niobium superconducting electrode should be clearly defined.
Currently, a system of Nb/Al/AlO.sub.x /Nb is known as the Josephson junction that satisfies the foregoing conditions.
FIG. 1 shows a typical structure of the Nb/Al/AlO.sub.x /Nb system in cross section, wherein the Josephson device includes a lower superconducting electrode 104 of niobium that is formed on a silicon substrate 102. On the upper major surface of the lower electrode 104, an aluminum layer 106 is deposited as an overlayer. The aluminum overlayer 106 has an upper major surface that is slightly oxidized to form a barrier layer 108 of AlO.sub.x. On the barrier layer 108, an upper superconducting electrode layer 110 of niobium is provided.
In the system of FIG. 1, the aluminum overlayer 106 covers partially the upper major surface of the niobium lower superconducting electrode 104 uniformly and without a pinhole even when the thickness of the layer 106 is in the order of 2-3 nm. The barrier layer of AlO.sub.x, formed on the surface of the aluminum overlayer 106, exhibits excellent current-voltage characteristics for the barrier of the Josephson devices.
In addition to AlO.sub.x, other materials such as tantalum o- zirconium are studied as a candidate for the overlayer. By oxidizing the surface of tantalum or zirconium overlayer, barrier layers having excellent characteristics are obtained. On the other hand, the attempt to form the Josephson junction that has the structure of Nb/NbOx/Nb has been failed. In this structure, the NbOx barrier layer is formed by the oxidation of the surface of the niobium superconducting electrode. It was found that NbO having the transition temperature of 1.4K is included in the NbOx barrier layer thus formed. In addition, oxygen in the NbOx barrier layer tends to react with niobium upon the barrier layer tends to react with niobium upon the deposition of the upper superconducting electrode on the barrier layer. Thereby, the property of the barrier layer is changed and the Josephson device having the above construction cannot exhibit the sharply defined threshold in the I-V characteristics.
In designing the Josephson device having the overlayer structure, one has to consider the so-called "proximity effect." The proximity effect is treated by the MacMillan's theory. Hereinafter, the proximity effect will be explained briefly with reference to FIG. 2.
Referring to FIG. 2, a superconducting layer S is provided in contact with a normal conduction layer N. The layer S has a thickness of d.sub.s while the layer N has a thickness of d.sub.n. Further, respective layers have energy gaps .DELTA..sub.s.sup.ph and .DELTA..sub.n.sup.ph. It should be noted that the energy gaps shown above correspond to the state wherein there is no proximity effect.
Now, the value of the energy gaps .DELTA..sub.s.sup.ph and .DELTA..sub.n.sup.ph is dependent on the lifetime broadening parameter .GAMMA..sub.s,n that is defined as: EQU .GAMMA..sub.s,n =h/.tau..sub.s,n =h.multidot.v.sub.F.sup.s,n .multidot..sigma./2Bd.sub.s,n ( 1)
where h stands for the Planck's constant, .tau..sub.s,n stand for the lifetime of an electron in respective layers S and N, V.sub.F stands for the Fermi velocity, d.sub.s,n represent the layer thickness of respective layers S and N, B represents a function related to the ratio between the electron mean free path and the layer thickness, and .sigma. represents the probability of tunneling.
Using Eq. (1), the energy gaps .DELTA..sub.n and .DELTA..sub.s for respective layers N and S under the presence of the proximity effect are obtained according to the following equation: ##EQU1## where, .DELTA..sub.n.sup.ph and .DELTA..sub.s.sup.ph represent the respective energy gaps for the layer N and the layer S without the proximity effect. E represents the energy.
From Eqs. (2) and (3), it can be seen that the proximity effect increases with a decreasing lifetime broadening parameter .GAMMA..sub.s,n, and the value of the energy gap .DELTA..sub.s (E) is decreased accordingly.
As can be understood from Eq. (1), the lifetime broadening parameter strongly depends on the Fermi velocity of the material forming the layers S and N, and one obtains a strong proximity effect when the material has a small Fermi velocity. Further, according to the Bardeen-Cooper-Schrieffer ("BCS") theory, it is known that the Fermi velocity is proportional to the coherent length. Thus, when a material having a small coherent length is used, the proximity effect would appear more strongly even when the layer thickness is held the game.
The condition that a Josephson junction having an aluminum overlayer exhibits excellent device characteristics is studied in detail. Briefly, the conditions are summarized as follows:
In order to satisfy the foregoing conditions, a fabrication process that provides minimum thermal as well as physical damaging to the layers deposited should be used. Because of this reason, a D.C. magnetron sputtering process is conventionally employed for depositing the niobium superconducting electrode and the aluminum overlayer.
Currently, an eight-bit digital signal processor is realized by using a Josephson integrated circuit that include about 6300 gates. In this integrated circuit, about 25000 Josephson junctions of the Nb/Al/AlO.sub.x /Nb construction are used. In designing future Josephson integrated circuits having 10,000 gates or more, on the other hand, various improvements are needed to reduce the problems such as disconnection of interconnection patterns or short coerced of the interconnection patterns, in addition to the improvement of the reliability of the Josephson junction itself. Such improvement includes the improvement in the reliability of peripheral processes.
Meanwhile, current integrated circuits of Josephson devices generally have a structure shown in FIG. 3 (A).
Referring to the drawing, the integrated circuit is constructed on a silicon substrate 122 and includes a niobium ground plane 124 formed on the upper major surface of the silicon substrate 122. On the ground plane 124, there is provided a silicon oxide insulation layer 126, and niobium electrodes 130a and 130b as well as circuit elements 128 such as molybdenum resistance element are provided on the upper major surface of the silicon oxide layer 126. In the illustrated example, the niobium electrode 130a is connected to the niobium electrode 130b via the molybdenum resistance element 128.
On the niobium electrodes 130a and 130b, there are formed AlO.sub.x barrier layer 132a and 132b respectively, wherein an upper niobium electrode 134a is provided on the AlO.sub.x barrier layer 132a and an upper niobium electrode 134b is provided on the AlO.sub.x barrier layer 132b. Further, the molybdenum resistance element 128, the lower niobium electrodes 130a and 130b, and the upper niobium electrodes 134a and 134b are covered by a silicon oxide insulation layer 136. The silicon oxide layer 136 is provided with contact holes that expose the upper niobium electrodes 134a and 134b respectively, and a niobium superconducting interconnection pattern 138 is provided on the silicon oxide layer 136 in contact with the niobium electrode 134a and 134b via the contact holes.
In the conventional Josephson integrated of FIG.3 (A), there is a problem of unreliable interconnection because of the irregular surface morphology of the silicon oxide layer 136. More specifically, the silicon oxide layer 136 includes a number of steps and these steps tend to cause disconnection of the interconnection pattern.
Because of this reason, an integrated circuit having a planarized structure is studied.
FIG.3 (B) shows a structure of the Josephson integrated circuit wherein planarized niobium electrodes 130a and 130b are used in place of the electrodes 130a and 130b. The structure shown in FIG.3 (B) may be easily fabricated by the planarization techniques that are used commonly in the semiconductor fabrication processes. More specifically, such a planarization technique may include processes such as etch back or bias sputtering.
When using these planarization techniques, one has to keep in mind that such a process may include a heat treatment process at 150.degree.-200.degree. C. In correspondence to this, the Josephson devices that are formed in the integrated circuit must endure the heat treatment conducted at about 200.degree. C.
The inventors have studies the temperature durability of the Josephson junction having the foregoing Nb/Al/AlO.sub.x /Nb structure.
FIG.4 (A) shows a voltage-current characteristic curve of the Josephson junction measured without heat treatment. As can be seen clearly in the drawing the Josephson device shows a clear gap voltage for the finite voltage state and provides substantial no output current as long as the device is in the finite voltage state. In the superconducting state, on the other hand, the device provides a finite current even when there is no voltage applied to the device. Thus, one can clearly distinguish the finite voltage state and the zero voltage state from the current flowing through the Josephson junction.
FIG. 4(B) shows the voltage-current characteristic curve of the foregoing Josephson device after a heat treatment conducted at 200.degree. C. As can be seen in FIG. 4(B), the characteristic curve is characterized by a ill-defined gap voltage for the finite voltage state, and the current flowing through the Josephson junction increases gradually with increasing voltage applied across the niobium superconducting electrodes. In such a device, a finite current flows through the Josephson junction and it becomes difficult to distinguish the state of the Josephson device from the current flowing therethrough. Thereby, the digital application of the Josephson device becomes considerably difficult.
The inventors have further studied the reason of this degradation of the voltage-current characteristic by the Auger electron spectroscopy and found a result shown in FIGS. 5(A) and 5(B), wherein FIG. 5(A) shows the structure of the Josephson junction and FIG. 5(B) shows the signal intensity representing the concentration of various elements as a function of the etching time.
Referring to FIG. 5(B), it can be seen that there appears a change in the concentration of aluminum and niobium particularly at the interface between the niobium electrode 104 and the aluminum overlayer 106 in the experiment for the as-formed device and in the experiment for the device heat treated at 200.degree. C. More specifically, the boundary between the niobium electrode 104 and the aluminum overlayer 106 is blurred somewhat after the heat treatment.
In order to confirm the foregoing finding, a series of measurements were made to confirm the distribution profile of various elements in the device for the samples that include the device as formed, the device applied with heat treatment at 200.degree. C., the device applied with heat treatment at 300.degree. C. and the device applied with heat treatment at 400.degree. C.
FIGS. 6(A) and 6(B) show the result of measurement, wherein FIG. 6(A) merely shows the structure of the device that is used for the measurement.
As can be seen in the distribution profile of FIG. 6(B), aluminum atoms in the aluminum overlayer 106 cause a diffusion into the underlying niobium electrode 104 with the heat treatment. With increasing temperature of the heat treatment, the diffusion is enhanced and the depth of aluminum reaching in the niobium electrode 104 increases.
The foregoing finding suggests that the diffusion of aluminum is caused due to the fact that aluminum is a typical low melting metal. It is believed that this diffusion of aluminum is caused due to the columnar texture of the niobium electrode 104. More specifically, the niobium electrode is formed from a number of columnar crystals aligned generally vertically to the surface of the electrode, and aluminum may penetrate into the electrode 104 along the grain boundary formed between the columnar crystals.
The foregoing result leads to the conclusion that, as long as aluminum is used for the overlayer, one cannot avoid the problem of degradation in the characteristics of the Josephson device and the Josephson integrated circuit having a planarized structure for interconnection cannot be achieved, as long as the Josephson device uses the aluminum overlayer.
There are investigations in search of Josephson junctions that show an improved durability against heat treatments. For example, Josephson devices that use a tantalum or zirconium overlayer have been proposed in combination with the niobium upper and lower electrodes. The barrier layer is formed by the oxidation of the tantalum or zirconium overlayer.
In the XPS (X-ray photoelectron spectroscopy) analysis, it is confirmed that these Josephson devices do show an excellent stability against the heat treatment.
FIGS. 7(A)-7(C) show the results of the XPS analysis for the device that uses a tantalum overlayer, wherein FIG. 7(A) shows the device structure, FIG. 7(B) shows the atomic concentration profile for the sample that is formed as it is, and FIG. 7(B) shows the atomic concentration profile for the sample that is heat treat at 250.degree. C. for 1 hour.
As can be seen in FIGS. 7(B) and 7(C), the distribution profile of tantalum does not change substantially even after the heat treatment. Associated therewith, there is no substantial change in the atomic concentration profile at the boundary between the tantalum overlayer and the underlying niobium electrode. The same result was obtained for the device that uses a zirconium overlayer.
About the possibility of use of the tantalum or zirconium overlayer for the Josephson junction, the inventors have made a more detailed examination by using the SIMS (secondary ion mass spectroscopy) analysis.
FIGS. 8(A) and 8(B) show the result of the SIMS analysis for the Josephson junction having the zirconium overlayer, wherein FIG. 8(A) shows the structure of the device and FIG. 8(B) shows the count of the secondary electrons for respective elements a function of the depth, measured from the top surface of the upper niobium electrode to the bottom surface of the silicon substrate. In FIG. 8(B), the curve designated as (A) represents the distribution profile of zirconium atoms for the device formed as it is, the curve designated as (B) represents the same distribution profile for the device that was subjected to the heat treatment at 200.degree. C. for 1 hour, the curve designated as (C) represents the same distribution profile for the device that was subjected to the heat treatment at 300.degree. C. for 1 hour, and (D) represents the same distribution profile for the device that was subjected to the heat treatment at 400.degree. C. for 1 hour. As can be seen clearly in the curve (D), the concentration profile has changed due to the heat treatment, indicating that the diffusion of zirconium has occurred.
FIGS. 9(A) and 9(B) show a result similar to FIGS. 8(A) and 8(B), wherein tantalum is used for the overlayer instead of zirconium. In this case, too, one can see a slight change in the concentration profile of tantalum with the heat treatment applied at 200.degree. C., 300.degree. C. and 400.degree. C. Similar to FIG. 8(B), the curve (A) shows the result for the device formed as it is, the curve (B) shows the result for the device heat treated at 200.degree. C., the curve (C) shows the result for the device heat treated at 300.degree. C., and the curve (D) shows the result for the device that was heat treated at 400.degree. C.
Further, the voltage-current characteristic of the Josephson junction was checked for the device of FIG. 8(A) and the device of FIG. 9(A). In this experiment, there was observed the degradation of the characteristic, similar to that shown in FIG. 4(B) for the device that was subjected to the heat treatment at 300.degree. C.
Thus, the conventional Josephson devices have the problem of durability against heat treatment, which is unsolved, and there is a demand for a device that shows stable and satisfactory characteristics even though subjected to the heat treatment that is generally used in the planarization process.
Meanwhile, in the Josephson devices in general, one can increase the operational speed of the logic devices or increase the signal to noise ratio when one can use a material having a higher critical temperature To for the superconduction transition. For example, when one can use a material having the critical temperature Tc that is higher than 10K, one can increase the energy gap of the material and hence the output current when such a device is operated at the liquid helium temperature. Further, when such a high Tc material can be used for the Josephson devices, it is expected that the cooling system, for maintaining the Josephson device and the integrated circuits in the low temperature environment necessary for the operation, would be simplified.
To the date, however, the fabrication of the Josephson junction that operates with satisfactory performance has been successful only in the system of Nb/Al/AlO.sub.x /Nb that uses niobium for the electrodes and aluminum for the overlayer. In this system, however, one cannot achieve the critical temperature Tc exceeding 10K.
As a material having the critical temperature Tc that exceeds 10K, several materials such as NbN or NbCN are known. Thus, there are attempts to fabricate a Josephson device by using these materials for the electrodes. For example, there is an attempt to fabricate a Josephson junction by providing a lower NbN electrode on an silicon substrate, forming a barrier layer of NbOx by oxidizing the surface of the NbN electrode by an r.f. plasma oxidation process, and providing an upper NbN electrode on the barrier layer. In this device, however, the NbOx barrier layer cannot cover the upper major surface of the NbN electrode without forming pinholes and no successful result is reported. Further, there are attempts to deposit amorphous silicon of MgO on the lower NbN electrode. In the latter case, one has to form the barrier layer of amorphous silicon or MgO with a thickness in the order of 1 nm or less. However, the technique to grow a layer of a first composition on an underlying layer of a different composition with such a small thickness and uniformity by deposition has not yet been established.